The present invention is directed to a method for packaging articles using shrink-wrap film, and particularly to an invention using pre-perforated film.
It is known in the art to overwrap articles in a web of heat shrinkable film to form a multipack package by separating a tube of such film wrapped around spaced groups of articles along a weakened zone by shrinking the tube adjacent the zone and then by shrinking the tube section formed thereby around the articles to form a package. See U.S. Pat. No. 3,545,165.
Previous methods of packaging such as the above have involved feeding the groups of articles into a heat tunnel in series, with the film wrapped around the articles from the leading edge of the group to the trailing edge of the group. FIG. 1 shows how this is typically accomplished. Groups G of articles A are placed spaced apart on a conveyor C. A layer L of film F (usually from a roll of film) is wrapped around the groups G with the film layer L continuously covering adjacent groups G.
The groups G are then fed on the conveyor into a heat tunnel T. Heat and (typically) forced air is applied to the junction J between adjacent groups, causing the film layer L to soften at the junction J and pinch off between the groups, at the same time shrinking tightly against the groups G as shown. This results in complete packages P of articles A, with the film shrunk about them. The closed ends E of the packages (known as “bulls eyes”) are at ends of the packages in the direction of travel of the conveyor (shown by the arrow).
An extension to the above apparatus is shown in FIG. 2. Here, parallel conveyors C1, C2, C3, etc. carry article groups G1, G2, G3, etc. into the heat tunnel, where the above-described heat-shrinking occurs. The parallelism improves total throughput.
The apparatus shown in FIGS. 1 and 2 has a number of disadvantages. In gathering of multiple articles A into the groups G (known as “pack patterns”), the continuous tube of film creates design challenges to support the groups from the underside while the tube of film is formed around the product. This is further complicated by product size changeover requirements. Theoretically, the conveyor C that transports the product pack pattern into the heat tunnel would have to change widths for each change in product size to accommodate the tube of film around the pack pattern.
In yet another variation, cut sleeves of film F are used, one sleeve per article group, instead of a continuous layer of film F over the groups G. However, the groups G are fed serially into the heat tunnel T with the articles A in each group G oriented in such a manner that the film F will be shrunk around each group G with the resulting closed ends E (“bulls eyes”) oriented transverse to the direction of travel of the conveyor C. To improve throughput, multiple parallel streams of articles A may be fed into the heat tunnel.
This apparatus, too, has disadvantages. Cutting the film into multiple streams can cause a loss in cutting efficiency. Narrow streams of film are generally more “stretchy” than one wide, non-split web of film. This varies film tension and can cause cutting problems and film alignment problems. Cutting (splitting) the film into multiple streams also requires that the apparatus guide each stream apart from each other so the streams do not stick together when processing through the heat tunnel.
There is a need for a method and apparatus of packaging that addresses the above problems.
When shrink wrapping parallel streams of product, a single web of film is wrapped around the packages. This web is perforated and partially slit to match the product streams. The product may be fed into the film shrinking apparatus in parallel streams to increase throughput. The same machine will often be capable of running a single stream of large packages, or multiple streams of smaller packages. A single large roll of shrink wrap film may be used. If the film is printed with graphics, the graphic pattern will match the number of streams of product being processed. In one type of product stream, the web of film is perforated between the product streams, forming a weakened area between the streams. The individual streams of packages must be separated into individual units. The weakened areas between the packages pull apart as the film shrinks in the heat tunnel.
The current standard is to completely slit the film into individual webs for each product stream. This requires that the webs be separated transversely. A spreader bar performs this function. The spreader bar must be adjusted for different stream patterns. The spreader bar adds drag to the film which causes web stretch, which can disrupt film registration. By perforating the film web instead of fully slitting it, the spreader bar and all of its issues can be eliminated.
The wrapper mechanism sometimes fails to place the film under a package. A photo-eye is used to detect the absence of the film as the wrapping wand carries the film over the top of the package. When multiple split streams are used, a photo-eye is required for each stream. When changing from single to multiple streams, the photo-eyes may have to be adjusted to align with the individual web paths. When using a single perforated web, the wrapping acts as if on a single sheet so that the sheet will entirely wrap or entirely miss. The absence of the sheet can be detected by one photo-eye which can be mounted in a fixed position.
As the wrapper wand places the film over the product, the film can become angled and not wrap squarely. The problem is worse when the film width is narrow compared to the length.
There is a need for an apparatus for creating perforations in the web of shrink wrap film prior to the wrapped articles entering the film shrinking apparatus.
It has been found that the film which is at the bottom of the package tends to weaken more slowly, sometimes preventing the packages from fully separating. The problem is reduced by fully slitting the film in this region before it is wrapped around the product.
There is thus a need for a film slitter for fully slitting the film that will encounter the bottom of the package before it is wrapped around the product.
U.S. Pat. No. 5,771,662 discloses a rotary cutter for cutting laterally extending cuts and perforations across heat shrink film. However, such a cutter is not usable in creating perforations in heat shrink film along the length of the film, so that the film separates along the perforations and shrinks against the packages.